U.S. Pat. No. 3,249,438 discloses an apparatus for removing contaminants e.g. water from a fluid such as oil. The apparatus comprises filters placed inside a vacuum chamber, and the combination of the vacuum and oil film on the outside surface of the filters removes water from the oil in the form of a vapour. The vapour is thereafter removed from the vacuum chamber. There are not described means for controlling liquid foam in the vacuum chamber.
U.S. Pat. No. 6,224,716 discloses an apparatus much like U.S. Pat. No. 3,249,438 but it also comprises a heating circuit for the oil to speed up the evaporation of the water from the oil. There are described pumps and valves for feeding liquid to the vacuum chamber and removing liquid from the vacuum chamber. There are not described means for controlling liquid foam in the vacuum chamber.
U.S. Pat. No. 5,574,214 discloses an apparatus for the treatment of transformer oil, in which the oil is treated by being passed through a vessel containing a conventional filter material and a molecular sieve. A molecular sieve is a material which is able to retain molecules of a specific molecular weight. The conventional filter material thus removes the particles from the transformer oil and the molecular sieve removes the water. This method is disadvantageous in that over time the molecular sieve is exhausted and has to be replaced.
International publication No WO 00/52445 discloses an apparatus, in which a filter cartridge is used to filter the transformer oil. The filter cartridge contains a filter material such as Fuller's earth. Fuller's earth is a type of soil, which inter alia is used as cat litter, and which possesses the ability to absorb water very easily. This device is encumbered by the same drawback as the above US patent, ie that the filter material is exhausted and has to be replaced occasionally. The WO publication further discloses a vessel provided with a vacuum pump, which is used to minimise the gas suspension in the transformer oil.
U.S. Pat. No. 2,062,934 describes a filter system where the liquid flows through disk-like filter elements from the outside of the filter elements to the inside, and where the filter elements are placed inside a vacuum chamber. It is mentioned that the liquid is the oil from a transformer and that the system removes water from the oil.
U.S. Pat. No. 6,224,716 describes a filter system for lubricating oils where cylindrical, coalescent filter units are placed in a vacuum chamber and where heated oil is passed from the inside of the filter units to the outside. It is specifically mentioned that the system can remove water from the oil.
DK patent No 156542 discloses a filtering device comprising a plurality of stacked filtering elements forming a filter cartridge. The disc-shaped filtering elements each has a central aperture forming a central passage with apertures to the inner cavity of the individual filtering elements in the stacked position of the filtering elements. Typically, the central passage has an inlet opening at one end and an opening sealed by means of a plug at the other end. Each filtering element has a planar outer side adjacent the central aperture and a corrugate side. As a result each pair of filtering elements opens into the central passage on one side, while closely abutting the adjacent pair of filtering elements on its inner side.